To pave the way for eliminating exchanges from incoming requests,
add simple inline fc_frame_sid() and fc_frame_did() functions
which get the FC_IDs from the frame header. This can be almost
as efficient as getting them from the sequence/exchange.

Move ntohll, htonll, ntoh24 and hton24 to <scsi/fc_frame.h>
since we need them there and that's included by <scsi/libfc.h>